1. Field of the Invention
The present invention relates to methods and apparatuses for processing a substrate, for example, wet processing of a semiconductor wafer in, for example, a scrubber, wherein both sides of a wafer are scrubbed.
2. Background Information
In the fabrication of semiconductor devices, numerous wet clean steps are performed on a substrate such as a semiconductor wafer at various stages throughout the process. In some cases these wet clean steps may comprise scrubbing the wafer with a brush, and may comprise one or more spin rinses wherein the wafer is sprayed with one or more cleaning solutions and/or water while the wafer is spinning. In many cases, only the front side of the wafer upon which devices are to be formed is cleaned. In many other instances, it is desirable to clean both sides of a wafer. For example, semiconductor substrate manufacturers typically scrub both sides at the completion of the slice manufacturing process. Additionally, other processes may require both sides of a wafer to be scrubbed. For example, chemical mechanical polishing (CMP), is carried out in a slurry, which is an extremely dirty environment by semiconductor fabrication standards. Therefore, both sides of the wafer must be scrubbed after the CMP operation to remove all contamination prior to subsequent processing. Finally, scrubbing, wherein both sides of a wafer are scrubbed, may be used in any application where it is desired to keep the back side (as well as the front side) of the wafer clean, for example, to reduce contaminants introduced into equipment and/or the process from the back side of the wafer.
A wet processing system such as a scrubber, wherein both sides of a wafer are scrubbed comprises several distinct stations or modules. Each module is typically enclosed in a box-like structure which comprises the appropriate processing apparatus for that station. For example, a scrubber, wherein both sides of a wafer are scrubbed may comprise a load or send station, one or more scrub stations, a spin rinse and spin dry station, and an output station. The load or send station typically comprises a platform for holding the cassette, an elevator for lowering and raising the cassette, and a sensor for sensing the presence of a wafer. Additionally, the load station may comprise sprayers to spray the wafers with filtered, deionized water (D.I. water) to keep them wet while they are awaiting processing. This is necessary where the previous operation, such as chemical mechanical polishing, leaves wet contamination (e.g., the slurry mixture) on the wafers, which, if dried, would be virtually impossible to remove. The scrub station typically comprises one or more brushes, wheels which grip and turn the wafer by its side during scrubbing, and sprayers or nozzles for dispensing chemicals. The spin station typically comprises a nozzle for a final water rinse, a spinner for spin drying, and a lamp to provide heat assisted drying. The spin station may also comprise a nitrogen blow off in addition to or in place of the heat lamp to assist drying. Finally, an output station comprises a platform for holding the cassette of cleaned and dried wafers. Additionally, one or more types of wafer transport mechanism, such as rollers, belt conveyors, robotic arms, etc., are provided within and between the stations to transport wafers within and between the stations. The system also comprises numerous sensors to detect the presence of a wafer, and to determine the location of the flat of the wafer, if desired. Often, the portion of the system up to and including the final scrub station is considered the "dirty" side of the system, while the portion after the cleaning stations, including the output station is considered the "clean" side of the system because the wafer has already been processed through a first and a second scrub operation. During the spin rinse and spin dry, the wafer is typically lowered into a cup-shaped structure having a vacuum pulled from the bottom, so that liquid spun off is not directed upward but rather is directed outward and downward. To maintain cleanliness, the top of the dry station is typically open to the fabrication area's overhead laminar flow, or may have its own laminar flow unit to maintain the cleanliness of the wafers. Typically, each module has one or more openings in the bottom portion to allow for connections to the appropriate facilities.
While systems have been developed to perform various wet processing operations such as the scrubbing operation, wherein both sides of a wafer are scrubbed, described above, many drawbacks exist in prior art systems. For example, as mentioned above, in the scrubbing operation, wherein both sides of a wafer are scrubbed operation, the substrates must be kept wet prior to and during the cleaning operation, requiring sprayers to keep the wafers wet. Because of the spraying, the sender station requires a lid or cover to prevent any liquid from spraying outside the station. Similarly, as the scrubbing operation is a wet process with various liquids being sprayed on the substrate and/or brushes, this station too requires a lid or cover. The cover has a square or rectangular cross section and is attached to the back of the machine by hinges. One difficulty with the cover is that if it needs to be lifted, liquid frequently drips off the lower front lip as it is lifted. One problem which occurs is that as the cover is lifted, the front lip is disposed over the edge of the enclosure of the station or may extend outside of the enclosure, causing liquid to drip on the external portion of the station, and/or the surrounding floor. The water that drips outside of the enclosure for that station may get into the machine, potentially causing some damage. In any event, water dripped outside of the enclosure typically increases the contamination level in and around the machine. Additionally, some dripping may occur within the station as the cover is lifted. Depending upon where and when this liquid drips, it may contaminate the wafer. A further problem is that some dripping may occur off the inner top surface of the cover when it is closed due to accumulation of fluid beyond what can adhere to the top surface without dripping. For example, in a scrub station, liquids, including solutions of various detergents or other chemicals may build up, concentrating contaminants, and eventually drip. The dripped on fluid may not be removed during the cleaning process, especially if the drip occurs after the last brush station, resulting in contamination of the substrate.
As a wafer is processed, it is passed from one station to next. For example, the sender sends a wafer to the first brush station where it is scrubbed. As mentioned above, the wafer may be sent by a conveyor belt type mechanism having belts which contact the wafer from the underside. As the belts turn, they move the wafer out of the cassette and toward the next processing station. Typically, the entire mechanism itself moves to bridge the gap between the two stations and pass the wafer to another conveyor belt mechanism in the next station. Alternatively, other similar mechanisms may be used. One problem that occurs in transporting the wafer is that as the wafer is wet, it may drip, causing fluid to flow in the interface between the two processing modules. Additionally, the transport mechanism may convey and drip liquid between the modules. The contribution from the transport mechanism may be minimized by, as described above, moving the entire transport mechanism to bridge the gap between stations when conveying a wafer, and then moving it away from the gap at all other times. However, this does not entirely eliminate dripping between stations from the conveyor mechanism. Additionally, this method may not be as convenient as placing a conveyor permanently between stations. Since many of the fluids may contain contamination from the wafers, or dissolved cleaning chemicals, as the dripped material dries, flaking is a problem. This increases the level of contamination, and may cause damage to the system. To remedy this, a thin flexible piece of a plastic type material is placed over the interface of the two enclosures. This method is not entirely satisfactory as some liquid may get underneath the plastic and between the two enclosures. Bacteria can easily grow in this still liquid and eventually get on the wafers as contamination. Additionally, the plastic may move or be ripped, etc.
In some cases, a wafer transport mechanism may move to a location to bring a wafer to a processing point and then move out of the way during processing. For example, a transport mechanism may move into position to transport a wafer from the final scrub station into the spin dry station. Once the transport mechanism has conveyed the wafer to a position where the fingers of the spindle wafer holder of the spin station can grasp it, the transport mechanism moves out of the way. The transport mechanism typically is coupled to a carriage which moves along a single rail. Often, the portion of the transport mechanism not coupled to the rail is extremely unstable due to the long cantilever bending moment, leading to misalignment of the transport mechanism relative to the fingers of the spindle wafer holder. This can cause a wafer to be dropped as the fingers attempt to capture a wafer while positioned on the transport mechanism.
As described above, in the load station the cassette of wafers is lowered until a wafer is sensed by a sensor. Since the wafer is in a stacked cassette, a sensor which detects an interruption in an optical pathway cannot be used. Rather, in this application, a capacitive sensor may be used, which detects an increase in capacitance when the wafer reaches a specified distance above the sensor. Alternatively, a reflective optical sensor having some type of sender and receiver which emits radiation, such as visible or infrared radiation, and receives reflected radiation from the substrate, may be used. This type of sensor depends upon receiving a signal above a certain threshold when the wafer is at a specified position. In the sender station, one problem that occurs is that water from the sprayer may be present on the sensor, leading to false results. In the case of the capacitive sensor, liquid on the surface or outer casing of the sensor may cause an increase in capacitance equal to or greater than that caused by the wafer. In the case of the reflective optical sensor, the liquid may act as a lens, either lengthening or shortening the light path, causing a wafer to be sensed too soon or too late. The sensing of wafer when none is present is problematic in that operations must cease, as the computer system expects a wafer to be present, and therefore assumes an error has occurred when no wafer is sensed at the downstream station.
In addition to the above-described sensor in the load station, other stations downstream of the sensor typically comprise one or more sensors to determine the position of the wafer, and/or for example to determine the position of the flat on the wafer as it is being processed. In the downstream modules, since a single wafer and not a stacked cassette, is to be detected, a through-beam optical sensor may be used to detect the presence of a wafer. The through-beam optical sensor comprises two components. A first component sends a signal such as light or infrared radiation. The first component is typically mounted to some part of the system's frame or to another mechanism. The second component receives the signal and is similarly mounted to the frame or another component. The second component is mounted opposed to the first component such that when a wafer is present between the two, its presence is sensed by the interruption of the signal. One problem with these sensors is that during construction of the system, or reassembly after repair or maintenance, the two sensors must be precisely aligned. Even if the two components are initially precisely aligned, they may become misaligned due to the movement of the parts of the system over time. In such a case, the receiver may not receive a sufficiently strong signal when no wafer is present so that a wafer is detected even in the absence of a wafer. Additionally, some of the sensors must be repositioned for different sized wafers, requiring an alignment operation each time wafer size is changed.
As described above, a system may comprise two brush stations. This is typically needed because the incoming wafers are extremely dirty, requiring the first station to clean the bulk of the material, and the second station to reduce contaminants down to extremely low levels required for satisfactory yields. One problem with the requirement of two scrub stations is that each station must be aligned to the proceeding and preceding stations, so that an increase in the number stations increases the complexity of the system. Additionally, the requirement of a separate module increases the number of parts and complexity of the system.
As is well known, in all processing systems it is desired to keep the wafer as clean as possible, and particularly in a system such as a scrubber, wherein both sides of a wafer are scrubbed, whose purpose it is to clean the wafer. In prior art systems, the system may contribute some particulates due to the layout of the system. For example, as described earlier, the bottom portion of the modules are open to allow for connection to the facilities in the lower portion of the system. As described earlier, there is typically a laminar flow flowing from above and then through the system, including through the processing modules, into the chassis, and then out the back and/or side of the system. Because of the numerous components, etc., much of the flow is not laminar in the lower portion of the machine but rather may comprise turbulent flow, diffusion effects, eddy currents, thermal currents, etc. This is particularly problematic if air from the dirty side, e.g. the scrub stations, mixes with the clean side, e.g. the output station. Because of the mixing and eddy currents, airflow beneath the system may transport contamination from the dirty side to the clean side. An additional system layout related problem is that contamination from the lamp and related structures may reach the wafer before, during or after drying. As noted earlier, a laminar airflow exists from above the wafer being dried. The heat lamp is disposed directly above the wafer, in the path of the laminar flow. As the lamp and/or wires heat up and cool down, and as the machine vibrates, particulates may be thrown off which then reach the wafer through the laminar airflow. It will be appreciated that contamination at this point is extremely detrimental as the wafer has already completed the cleaning process.
A wet processing system such as a scrubber, wherein both sides of a wafer are scrubbed, typically comprises numerous chemicals with which to clean and scrub the wafer. For example, various solutions of detergents, solvents, acids, bases, and surfactants, as well as D.I. water, are flowed through one or more nozzles. Each of these is controlled by a flow controller which is typically coupled to a flow meter. The flow controllers and meters are usually disposed on a panel in an easily visible section of the system, so that the flow may be readily monitored. However, the connections to the flow meters are on the backside of the panel, internal of the machine, so that the machine must be opened up to connect the chemicals. This can be a time consuming process, as each input and output line must be connected to the correct flow meter so that appropriate chemicals are flowing to the appropriate nozzles, and so that the flow controllers and meters are controlling and measuring the fluid for which they labeled. This is a difficult task because as the flow controllers and meters are outside of the machine, and the appropriate connections are inside the system, one cannot see the flow controller to which each backside connection belongs. Obviously, an incorrect connection may cause damage to processed wafers until the error is discovered.
What is needed is a system and apparatus which provides for processing of a wafer without adding contamination through, for example, dripping from the cover, leakage between modules, contamination from laminar airflow mixing, and contamination from a structure disposed above the processing apparatus such as a heat lamp. What is additionally needed is a system having a method and apparatus for sensing a wafer in a wet environment which may reliably detect a wafer even in the occasional presence of water built-up on the sensor. Additionally, it is desirable to provide sensors for sensing a wafer which are not subject to misalignment over time, and which may be easily positioned to accommodate different size wafers, or different sensing locations without requiring alignment. What is further needed is a method and apparatus providing for stable operation of moveable transport mechanisms. Further, what is needed is a method and apparatus for reduced components and reduced alignment between modules in a system having several modules. What is also needed is method and apparatus for connecting fluids to the system which may be done easily, with reduced risk of incorrect connections.